Helix assembly



W. H. THON HELIX ASSEMBLY Oct. 12, 1965 4 Sheets-Sheet 1 Filed May 1. 1961 R m m W.

WILLIAM H. THON ATTORNEY W. H. THQN HELIX ASSEMBLY Oct. 12, 1965 4 Sheets-Sheet 2 Filed May 1, 1961 INVENTOR.

WILLIAM H. THION ATTORNEY Oct. 12, 1965 W. H. THON 3,211,945

HELIX ASSEMBLY Filed May 1, 1961 4 Sheets-Sheet 3 INVENTOR.

WILLIAM H. TH ON TTORNEY Oct. 12, 1965 w. H. THON 3,211,945

HELIX AS SEMBLY Filed May 1, 1961 4 Sheets-Sheet 4 INVENTOR. WILLIAM H. THON ATTORNEY United States Patent 3,211,945 HELIX ASSEMBLY William H. Thou, Los Altos, Califl, assignor to Sylvania Electric Products Inc., a corporation of Delaware Filed May 1, 1961, Ser. No. 106,806 9 Claims. (Cl. 3153.S)

The present invention relates to traveling wave tubes and particularly to an improved envelope and mount for the helical conductor for such tubes.

In a conventional traveling wave tube, a gun assembly produces and projects an electron beam along the longitudinal axis of an elongated helical conductor, called a helix, at the proper velocity for interaction with the electric field component of the high frequency signal traveling along the helix. Adjacent to the signal output end of the helix is a collector upon which the electron beam impinges after passing through the helix. The gun assembly, helix and collector are enclosed by a vacuum-tight envelope through which coupling is made of the radio frequency signal wave from outside circuits to the ends of the helix. The entire tube is normally mounted in a suitable magnetic field coil, or permanent magnet, to prevent divergence of the electron beam.

In order to achieve maximum effective interaction between the electron beam and the radio frequency wave on the helix, accurate axial alignment of the helix and the electron gun is required. Misalignment causes the electron beam to strike the helix which reduces current density of the beam and heats the structure unduly. Ideally, the electrons in the beam should flow as close as possible to the helix without interception and for the entire length of the helix. Physical alignment of the parts of the tube is therefore critical in the assembly of the tube.

One aspect of this alignment problem is supporting of the helix in such a manner that its axis is a straight line. Also, the radio frequency loading effect of the helix support on tube performance imposes limitations on the kind of support used. It is with this part of the alignment problem that this invention is concerned.

In the past, the practice has been to locate rod support members in circumferentially spaced positions about the helix in the annular space between its outer peripheral edges and the inner surfaces of the envelope. These rod supports advantageously place a minimum of material in proximity to the helix to reduce the loading effect, and also serve to guide insertion of the helix into the envelope during assembly.

To serve as guides, however, the rods are usually secured to the helix in a separate assembly step, by glazing or brazing. This subassembly is then inserted into a loosely fitting glass or quartz envelope which houses the elements of the tube. To prevent movement of this assembly relative to the side wall of the envelope, the envelope portion adjacent the helix and rod assembly is usually evacuated and then heated until the glass softens sufficiently that the atmospheric pressure moves the glass walls inwardly against the rods. To effect a rigid helix mount, however, this operation must be closely controlled as to time and temperature. If, in heating the glass, the walls of the envelope are allowed to exert excessive radial pressure upon the helix, the pitch of the latter is distorted. Conversely, if the contraction phase is inadequate, the inside diameter of the envelope is oversize, and subassembly is free to move axially incident to vibration or shock.

Another method of mounting a helix is similar to that described above except that use is made of a glass envelope having an inner surface which is precisely formed to provide a reference or guide for accurate alignment of components in the envelope. The difference in assembly steps is that of inserting the rods and helix bundle into the tube with sufiicient longitudinal force to overcome the 3,Zll,945 Patented Oct. 12, 1965 binding between contacting surfaces. After the rod and helix subassembly is so inserted, it is snugly supported by the tube and there is no need for additional mechanical restraints.

An additional method deviates slightly from the above two in that the helix is mounted directly into a straight and vacuum-tight envelope having preformed internal ridges or flutes to contact peripheral edges of the helix. Still another method of mounting the helix within the glass envelope is to form ridges circumferentially of the helix with the helix in place, as shown in Best, Patent Number 2,206,369.

The disadvantages of these methods of mounting the helix within an envelope are as follows:

(1) With regard to inserting the helix into an envelope, if the glass envelope is pre-shrunk, i.e., shrunk on a mandrel to the tolerances required for subsequent assembly of the helix rods precisely within the envelope, the pitch of the helix may be distorted during the insertion of the helix and rod assembly.

(2) If the glass envelope is shrunk with helix and rod already inserted, the pitch of the helix may be distorted, as described above. Since the phase velocity of the radio frequency wave is a function of the pitch, any change results in non-synchronization of the beam and wave with a subsequent inefficiency of tube operation.

(3) The glaze used to fix the rods to the helix, in the preassembly operation, adds dielectric loading to the helix. This means tube power is dissipated in the glaze and tube performance degenerates.

(4) When glass tubing is used for the envelope, the steps needed to bring its straightness and inside diameter within operation specifications are time consuming and costly. Many envelopes are broken. If metal tubing is used, as the vacuum-tight envelope, the step of inserting the rod-helix assembly without distorting the pitch of the helix usually necessitates the envelope being mechanically deformed. This brings about problems in the fabrication of such assemblies, namely in providing sufficient clearance for helix assembly insertion without exceeding the elastic limit of the tube.

(5) It is extremely critical that uniform radial pressure forces bear upon the helix, since subsequent vibration can change the pitch of the helix, distorting the phase velocity as well as changing the dielectric loading. In addition to being vulnerable to vibration, a non-uniformly supported helix also causes non-uniform dissipation of heat generated during operation of the tube. The result is hot spots in the envelope which deform the glass and bring about misalignment of the assembly or even cause a loss of vacuum.

(6) A glass envelope or barrel is fragile and therefore requires a separate carriage to support it in the magnetic coil. The carriage comprises a long sleeve around the barrel and a pair of axially spaced annular supports between the sleeve and barrel which may also serve as a radio frequency coupling means for the helix. Such radio frequency couplers usually complicate adjustment of the magnetic focusing system and may required additional machining to allow such adjustment.

A general object of this invention is the provision of an improved traveling wave tube helix mount which avoids or minimizes the above disadvantages.

Another object of the invention is the provision of a helix mount which assures straightness and rigidity of the helix. Another object is the provision of a helix mount which locates minimum material in the proximity of the helix. An additional object of the invention is the avoidance of mechanical interference between the focusing magnet and the input and output radio frequency couplers for the helix.

Another object of this invention is to provide efficient and economical means for cooling the helix of a traveling wave tube. A further object of the invention is to provide an improved method of fabricating such a helix mount in which the above objects are achieved readily and at a low cost.

In accordance with the invention, the helix of a traveling wave tube is mounted coaxially within a tubular envelope, the wall of which comprises a plurality of longitudinally joined elongated tubular members. The helix is rigidly supported within this envelope by a plurality of straight longitudinal ceramic or quartz rods snugly fitted within cusps or grooves formed between adjoining tubular members. All grooves formed between the members are dimensionally equivalent because the diameters of the tubes are identical and the tubes are located an equal distance from the axis of the envelope. Uniform pressure is thus applied to the helix throughout its length by rods whose outer surfaces make contact with the helix and which conform to the wall surfaces of individual tubular members comprising the envelope. Circumferential, radial and axial movement of the rods relative to the envelope is eliminated because each rod is essentially wedged between two of the outer members, there being extended contact between adjacent tubular members and each rod and between the helix and each rod. In addition to providing firm rod support, the effect of these contacts is to provide greater heat dissipating area of the rods.

The space adjacent the helix is utilized by locating the coaxial radio frequency input and output lines within two of the tubular members forming the envelope. The configuration uses the wall of the tubular member as the outer conductor of the line and an inner conductor coaxial of the axis of the member; the latter is secured to the helix at one end and to a coupling connector at the other for injection or removal of a radio frequency signal. The walls of the remaining tubular members facilitate the passing of cooling liquids or gases, entrance and removal of which is by tubular fittings located on the ends of two of these members.

The features of the invention will be better understood from the accompanying drawings in which embodiments of the invention are illustrated. In these drawings:

FIGURE 1 is a perspective view, partially cut away, of a traveling wave tube embodying the invention;

FIGURE 1a is an enlarged view of portions of the cut away to show the inside construction of the tube;

FIGURE 2 is an enlarged section of the helix mount taken on line 22 of FIGURE 1;

FIGURE 2a is an enlarged view of portions of FIG- URE 2 illustrating the construction of the helix support;

FIGURE 3 is a section similar to FIGURE 2 showing a modified form of helix mount in which the helix is supported directly on the multi-tube envelope wall;

FIGURE 4 is a perspective view, partially cut away, of one of the envelope members showing portions of the tube envelope;

FIGURE 4a is a partial longitudinal section along lines 4a-4a of FIGURE 4 showing coupling details;

FIGURES 5, 6, 7 8 and 9 illustrate steps in the method of construction of the helix support wherein FIGURE 5 is a perspective view, partially exploded, showing a portion of the tube envelope;

FIGURE 6 is a transverse section of the subassembly showing a partially completed envelope within a brazing fixture;

FIGURE 7 is a section of the entire envelope assembly within a brazing fixture;

FIGURE 8 is an exploded view of FIGURE 2 omitting the helix; and

FIGURE 9 is a transverse section of the modified structre of FIGURE 3 and a brazing fixture, illustrating a method of assembling the tube parts.

A preferred embodiment of the invention is illustrated in FIGURES 1 and 2 as a traveling wave tube comprising an elongated envelope 1 having reduced barrel portion 2 enclosing the helical conductor or helix 4 of the tube. The barrel portion 2 of the tube is of special construction described below to support the helix. Located in enlarged portion 13 of the envelope is the gun structure 14 which produces an electron beam which is then focused and accelerated along tube axis A through helix 4 to collector 5 at the opposite end of the tube. The electrons interact in a well-known manner with elec tromagnetic waves on the helix so that a signal applied to the input line 25 is amplified when it appears at the output line 26. The efficiency of the tube depends in large measure on the non-dispersive axial travel of the electrons through the helix 4. Since straight line flow is easiest to control, it is important in achieving high operating etficiencies that the helix itself be supported such as the present a precisely straight line axis. This invention concerns such a helix support and its assembly.

In a preferred embodiment, the barrel 2 of the tube envelope comprises six longitudinal tubular members 7, 8, 9, it Ill and 12 having straight axes which are parallel to each other and to the axis A of the tube. The tube members preferably have the same diameter and each makes longitudinal line contact with the two adjacent members so that the peripheries of each pair of adjacent members merge at their junction to form a cusp. These cusps are indicated at 15, 16, 1'7, 22, 23 and 24 in FIG* URE 2. The tube members are secured and sealed together by brazing along their lines of contact for the full length of the barrel to provide a vacuum-tight multi-tube wall for containing the helix structure. For purposes of locating minimum dielectric material in proximity of the helix, rods 18, 19 and 20 are circumferentially spaced 120 degrees apart in the cusps 22, 23 and 24 in longitudinal contact with the helix 4 and barrel 2. Each rod is snugly wedged in its support, i.e., cusp 22, 23 or 24 and exerts pressure on the helix which prevents its axial, radial or circumferential movement.

This is shown in FIGURE 2a, where rod 20 greatly enlarged is shown mounted within the cusp 24 formed between adjoining tubular members 7 and 11 in longitudinal or line contact with these members and helix 4. This arrangement achieves mechanical stability to incident shock loads because the forces acting on rod 20 are coplanar and balanced. That is, the direction and magnitude of the forces exerted by the outer surfaces of members 7 and 11 upon rod 20 (i.e., along line contact 36 and 35, respectively) and the direction and magnitude of the force exerted by the outer surface of the helix upon the rod (i.e., line contact 38) pass through axis 31 of the rod 2% in the same plane and have resultants which are equal and opposite. The result is an equilibrium of forces about the rod 20 along line contacts 35, 36 and 38, the components of which can be resolved in directions which oppose shock loads which may act upon the structure during operation.

The absolute magnitude of these component forces, however is a function of the diameters of the rods, helix and tubular members which make up the helix mount. For example, if the peripheral surfaces of rods 18, 19 and 20 are not wedged securely in the grooves 22, 23 and 24 formed between tubular members, there will be a reduction of the reaction forces incident on the helix. Most important, the helix may become misaligned. Therefore, control of the outside diameter of the rods, helix and tubular members, and the physical relationship of the parts, i.e., precise ratios between the diameters of the rods, the helix and tubular members, are essential to achieve uniform radial pressure upon the helix.

The former is manufacturing detail; while the ratios of the diameters of the parts are based on certain electrical parameters desired of the finished tube, and relate to the pitch and diameter of the helix (d FIGURE 2A) and the minimum distance X (FIGURE 2) from axis A of the tube to the outer surface of the tubular members. These parameters are (1) frequency and gain characteristics of the tube in the first instance, and (2) the radio frequency load due to the metal envelope located coaxially of the helix in the second.

It is also noteworthy that dimension X fixed by the above noted electrical requirement, in turn fixes both the diameters of the tubular members Which form the envelope (d and the diameters of the rods which support the helix (d the former (d by the mathematical principle that six circles of equal diameters can be circumscribed about a seventh circle of the same diameter, with the result X: /2d and the latter (d by the relationships of known dimensions as shown below, Equation (1), i.e.

The constant term K in the above formula, e.g., 1.02

in the instant embodiment, increases the diameter of rods 18, 19 and 20 in order to achieve uniform radial pressure upon the helix as explained thereinafter.

Adjacent rods 18, 19 and 20, external of cusps 17 and 22 and 15 and 23 but integral within tubular members 10 and 12, are located compactly formed input coaxial line 25 and output coaxial line 26. These lines facilitate the coupling of radio frequency energy into and from the helix without occupying excessive space about the helix and comprise outer conductors 27 (part of the hollow wall of the envelope) and inner conductors 33 which are located on the axis of the member tubes housing the lines. Support members 28 properly space the outer and inner conductors of these radio frequency lines.

In order to electrically connect each inner conductor 33 to the helix 4, a bend 29 (see FIGURES 4 and 5) is formed on the end of conductor 33 adjacent the helix, forming folded portions 61 projecting inwardly to the helix through opening 37 in the outer conductor and space 55, defined between the end of support member 28 and second support member 67 (FIG. 4A).

At the opposite end of the assembly, the coaxial lines project beyond collector 5 forming the end wall of the tube, where coaxial window members (FIGURE 4) comprising an opening through which the inner conductor projects are positioned in contact with the bore of the outer conductor to form a vacuum end wall. Adjacent windows 30, connectors 41 are mounted to the exterior of outer conductors 27 to which coaxial cables 43 are secured to couple the tube to external energizing circuits (FIGURE 1).

Tubular members 7, 8, 9 and 11 which are not used as coaxial input or output lines also project beyond the collector 5 and serve as parts of a cooling system for removal of heat from the helix and associated parts. These four tubular members are interconnected at their ends by means of suitable circumferential holes or passages (not shown) in the collector 5 and the end plate 108 of the gun housing so that cooling fluid from an external source, not shown, may be circulated over the length of the barrel to remove heat generated during operation of the tube.

Connection to an external manifold (not shown) is made by hose and coupler arrangement 144 adjacent plug members 44 in tubes 7 and 8 and attached to fittings 45 at the ends of tube members 9 and 11 for injection and removal, respectively, of the cooling fluid.

Passages in the collector 5 allow the cooling medium to flow from tube 7 to tube 8 while, at the gun end plate 108 of the gun housing, similar passageways allow the cooling liquid or gas to flow from tube 7 to tube 11, and from tube 8 to tube 9.

The entire helix mount consisting of the rnulti-tube barrel, the ceramic rods 18, 19 and 20 and the helix 4 is a rigid subassembly adapted to join with the other parts of the traveling wave tube. The barrel is joined with the enlarged portion 13 of the envelope as described below so that the axes of the gun 14 and the helix are precisely aligned, and the electron stream travels from the gun to colletcor 5 along the straight barrel axis.

By way of example, a helix mount embodying the invention and having the following dimension has been constructed:

Item:

Helix mount 6-- Barrel portion 2:

Dimension, inches Modification A modification which better adapts the above structure for use with permanent magnets as well as in a very high (1600 centigrade maximum) ambient temperature is shown in FIGURE 3.

Tubular members 150, 151, 152, 153, 154 and 155, forming the barrel wall, are made of ceramic material, such as a high alumina body and are mounted in a cluster about and in direct contact with the helix eliminating the rods 18, 19 and 20 of the previous embodiment.

Helix 4 is directly supported along its entire length by the tubular members comprising the envelope. This arrangement of the circumferentially spaced tubes making six individual line contacts With the helix gives maximum mechanical strength with minimum of dielectric material placed adjacent to the helix, the latter being greater than that of the preceding embodiment, however. Two of the tubular members, 152 and 154, contain coaxial lines 157 and 158 comprising outer conductors 159 and inner conductors 161, the latter extend through opening in the ceramic and metal walls for connection to the helix much the same as in the preceding embodiment.

As described in the preceding form, longitudinally extending fittings may also be provided on the free ends of two of the empty tube members and circumferential passages formed within the end walls so that cooling fluid from an external source, not shown, may be circulated over the length of the barrel to remove heat generated during operation of the tube.

Among the advantages of the resulting structure is high temperature operation and the small circular cross section of the configuraiton as a result of elimination of the ceramic spacer rods, viz., rods 18, 19 and 20 of FIGURE 2. In regard to the latter, the smaller cross section allows the size of the magnets used to focus the beam to be reduced which in turn reduces the weight of the all-over tube. If the diameter of the barrel is decreased by onehalf of its former diameter by adjusting the ceramic rod configuration, the weight of a tube would be approximately one-half of its former weight. The saving of such weight is of paramount importance to the design of equipment used in airborne application, affecting both the performance of the aircraft and its range.

In regard to the former, the ceramic envelope allows a helix to be processed and operated at much higher tem- 7 peratures than a similar helix mounted in a glass envelope. In addition, the undulated surfaces (grooves) of the envelope adjacent the outer peripheral edges of the helix allow removal of impurities liberated from these edges during the high temperature processing cycle.

Method The method of making the helix mount described above and generally indicated at 6 in FIGURES 1 and 2 will now be described with reference to FIGURES 4, 4a, 5, 6, 7 and 8.

The outer surfaces of the six members 7, 8, 9, 10, 11 and 12, which members preferably are made from molybdenum, are first ground to finished diameters approximately twice the helix diameter. The tolerance of each diameter is controlled within $00001 inch by using the centerless grinding techniques as described in the Tool Engineers handbook, 1st edition, McGraw-Hill, Inc. 1949, pages 905 to 906.

After all of the tubular members are so ground, members 10 and 12 are converted to coaxial lines suitable for carrying radio frequency energy to and from the helix.

Openings 37 are drilled in the Walls of the tube members at axial locations determined by the relative position and length of the helix to be used, and thereafter the inner surfaces of the tubes, bores 56 of tubes 10 and 12, are accurately machined as by broaching, to meet coaxial line specifications.

Inner conductors 33 are next accurately axially supported vwithin tube bores 56 by ceramic sleeve-like spacers 28 having central bore 51 through which the inner conductor extends. Prior to insertion of inner conductor 33 into each spaced bore, one end 32 of the inner conductor is bent radially outwardly as shown (FIGURE to permit that conductor end to project through wall openings 37 for ultimate connection to the helix. Each inner conductor-spacer subassembly is then slipped into one of tubular members and 12. The radial end portion 61 of the inner conductor may be longer than the radii of tubes 10 and 12 so that portion 61 is depressed to an acute angle with respect to the axis of the center conductor during the insertion step. When conductor portion 61 ultimately becomes aligned with wall opening 37, it springs back to its radial position through the opening. Ceramic plug 67 and metal plug 67a inserted into the respective bores of the tubular members secure the bent inner conductor and facilitate the impedance match between the helix and the coaxial line, see FIGURE 4a.

The vacuum-tight envelope comprising tubular members 7-12 are next assembled around the helix (1) by mounting two tubular members with their axes parallel forming a tube pair, (2) locating a ceramic rod in the cusp or groove between the tube pair forming a rod-tube subassembly, and (3) arranging three rod-tube subassemblies as a cluster about a helix to complete the envelope.

In order to explain more clearly the successive steps in the assembly operation, tubular members 7-12 are identified in pairs, to wit: tubes 8 and 10 comprise tube pair 47, tubes 9 and 12 form pair 48, and tubes 7 and 11 form pair 49, see FIGURE 8. It is pertinent to note that tubular members 10 and 12 which comprise the input and output coaxial lines are slightly longer than the remaining members which form the envelope to allow the attachment of the lines without mechanical interference with adjacent members.

In the fabricating sequence, to form tube pairs 47, 48, and 49, two tubular members (e.g., members 8 and 10) are placed lengthwise in a brazing fixture generally indicated at 70 in FIGURE 6 which comprises a base block 71 having a length equal to or greater than that of helix 4. To differentiate fixture 70 from fixtures subsequently in this process, it is hereinafter identified as the first fixture.

Lengthwise across the top surface 72 of base block '71 is a V groove 73. A precision ground silica rod 77 tangentially engages groove surfaces 75 and 76 along lines 78 and 79, respectively, near groove apex 74, and has a diameter equal to the diameter of each of members 712. Surfaces 75 and 76 are also precision ground, deviating from perfect flatness through their entire length by only 0.0001 inch T.I.R. (Total Indicator Reading), a requirement necessary to insure straightness of the subsequent envelope as will be explained hereinafter. An additional characteristic of the fixture is that the material must be of low expansion type, preferably a silica refractory to allow the fixture to be heated without appreciable deformation.

Members 8 and 10 are located within groove 73, above, and in engagement with rod 77 and surfaces 75 and 76. Members 8 and 10 tangentially contact the surfaces 75 and 76 at lines 80 and 81, the surface of rod 77 at lines 82 and 83 and each other along line 84. However, because of normal tolerances in the manufacture of rod 77 and members 8 and 10, the latter may deviate from actual contact with the groove surfaces and with each other at some portions of their length. Such contact is restored in subsequent steps, however, as will be explained below.

After a braze alloy 85, such as nickel and gold, is placed at junction 84 (used hereinafter to permanently fuse the tubular members), the assembly of rod and tubes is compressed to snugly contact the side walls 75, 76 of the fixture by application of rectangular top block 87. The compression by top block 87 is accomplished by abutting surface 88 in its central portion 89 which applies distributed pressure forces in a downward direction; this action forces tangential contact of the specified members of the assembly over its entire length.

The term downward as used herein refers to movement from the upper side of the fixture in FIGURES 6 and 7 towards its base block; similarly upward refers to movement from the opposite side of the structure towards the top block of the fixture. Likewise, the term forward or top refers to the upper surfaces, walls, etc., of the parts of these fixtures; similarly bottom, rear refers to the lower portions.

The abutting surface 88 of the central portion is shaped to tangentially contact the adjoining surfaces of the tubular members along longitudinal lines 92 and 93, forcing these rods to conform downwardly to the walls 75 and 76 of the groove. If the weight of this member is not sufficient to accomplish the straightening of the tubular members, an additional weight 94 may be located on the upper surface 68 of the top block 87. When the top block 87 and weight 94 are located as described above, no contact other than by surface of the central member 1s permitted because of the height of steps 95 and 96 separating the central portion from the exterior side portions 97 and 98 of the top block 87. After firm, coextending contact of the wall surfaces is made, the fixture containing the tube assembly is sent through a furnace at 980 centrigrade for 10 minutes in a wet hydrogen atmosphere to melt the braze alloy 85 and permanently secure the adjoining surfaces of the tubular members 92 and 93. After cooling, the completed tube pair is then removed and the process repeated for tube pairs 48 and 49.

Next, rods 18, 19 and 20 having thin layers of carbon disposed about portions of their outer surfaces to provide attenuation of unwanted signals, are attached to the longitudinally extending cusps or grooves 22, 23 and 24 formed at the junction of the tubular members.

In order to explain more clearly the successive steps that follow, tube pairs 47, 48 and 49 having rods 18, 19 and 20 mounted therewith, are identified as rod-tube subassemblies, to wit:

The subassembly comprising rod 18 mounted in cusp 22 between tube pair 47 is rod-tube subassembly The subassembly comprising rod 19 mounted in cusp 23 between tube pair 48 is rod-tube subassembly 111;

The subassembly comprising rod mounted in cusp 24 between tube pair 49 is rod-tube subassembly 112, shown in detail in FIGURE 7.

Two such subassemblies 110 and 111 are then placed in a second brazing fixture generally indicated at 69 in FIGURE 7. The side walls and 76 of groove 73 of base block 71 contact subassembly 110 along contact lines 113 and 114, and subassembly 111 along contact lines 106 and 107. These subassemblies contact silica rod 77, located at the apex of the groove along contact lines 99 and and each other along contact line 57. The subassemblies are thus located such that the line of tangency 57 of the subassemblies is parallel to the axes of the silica rod 77, and to surfaces 75 and 76. This also applies to the axes of the four tubular members. In addition, the axes define two planes with intersect at the axis of rod 77 at exactly 60 degrees.

After the subassemblies are wedged securely in place, a braze alloy 116 similar to one used previously is then placed in the cusp 117 formed along line 57 of the contacting pairs of subassemblies. Thereafter, a helix 4 of the correct diameter and of the correct pitch i.e., number of turns per axial inch, for maximum efiiciency of tube operation is gently placed in contact with rods 18 and 19 along tangential lines 62 and 63. The ends of the helix are then fused to the terminating ends 32 of the inner conductor 33 by heliarc welding or brazing procedures.

The third rod-tube subassembly 112 is then located in close proximity with the other assemblies and in contact with the helix 4, actually touching subassembly along line 121 and the helix 4 along line 123. A small longitudinal gap remains at 122 between members 11 and 12 caused by having the diameters of rods 18, 19 and 20 oversized, in accordance with Equation (1), above. Braze alloys 124 and 125 (nickel and gold) are placed along lines 121 and 122 of the assemblies to form the means of fusing these assemblies into a vacuum-tight envelope when the suspension is fired in a furnace under the conditions previously described. Thereafter a second silica rod 128 sirnilar to rod 77 is placed atop the third subassembly 112 in groove 129 to facilitate later steps as explained below. Sufficient to say, the configuration thus formed resembles an equilateral trapezoid in cross section, the upper end of which is the silica rod 129 and the lower end of which is silica rod 77.

The portion of the helix brazing assembly which protrudes above lower edges 119 and 120 of second fixtures 69 is then secured by top block 130 having a V groove 86 identical to that of groove 73 of fixture 70. The walls 90 and 91 of groove 86 tangentially contact the surfaces of subassembly 112 along lines 132 and 133, and the upper tubes 10 and 12 of subassemblies 110 and 111 along lines 134 and 135. The width of the top block is controlled to allow surfaces 136 and 137 of the exterior sides to snugly contact steps 138 and 139 which separate ledges 119 and 120 and upper surface 72 of the base block. Rods 18, 19 and 20 are flexible and will conform to their respective grooves, whose straightness was assured by the first fixture 70. The helix will be under compression by the rods because of the small gap 122 and the weight of the block 130. The result is a firmly secured straight helix.

After the parts are wedged within the confines of the fixture, the fixture is passed through the furnace under similar conditions as described previously to fuse the rodtube subassemblies at the intersections of their respective joints, viz., lines 57, 121 and 122. After the assembly is removed from the furnace and cools, the finished helix assembly is removed from the fixture and radio frequency and cooling means attached as explained hereinafter.

At this stage of the process the support has several desirable features whtich need enumeration. The weight of the top block 130, and the accuracy of the brazing fixtures 69 and 70 are constant, which facilitates manufacturing of similar helix structures in large quantities (i.e., constant helix pressure distribution from tube assembly to tube assembly). Each turn of helix 4 is similar to its neighbor, being uniformly and rigidly supported throughout the entire length of the helix by rod supports which in turn are firmly held in the cusps formed between the tubular members which form the envelope. The former is achieved by using identical tubular member having uniform diameters; and the latter is effected from the large effective wall thickness of the envelope, the effective wall thickness being considered to be equal to the diameter of the tubes. Assemblies of this nature are also fabricated easily and quickly without locating extensive material adjacent the helix, and, in addition, allow the barrel to be used for several purposes. Before these purposes are realized, however, several additional fabrication steps are necessary to assemble the parts into the composite tube shown in detail in FIGURES l and 2.

These steps involve mounting a tightly fitting collector 5 within the barrel wall beyond the output end of the helix. Next end plate 108 of gun enclosure 13 is placed against the other end of the barrel. Both parts are permanently affixed by vacuum-tight brazing joints.

Radio frequency connectors 41 are then permanently mounted on the ends of the radio frequency lines to provide means of injection and removal of radio frequency energy to and from the helix.

To use the remaining tubuar members of the envelope 7, 8, 9 and 11 as a closed cooling system, fittings 4-5 and end plug members 44 are then attached to or within the ends of these members as explained above.

Modification A further modification of an embodiment of the method according to the invention is illustrated in FIGURE 9.

Important features are the substitution of ceramic envelope members for the metal tubular members, and the elimination of the equally spaced helix support rods used in the previous embodiments. These changes reflect in distinct structural advantages: the latter allows the ceramic members to be located in direct contact with the helix decreasing the outside diameter of the finished envelope, while the former increases the electrical and mechanical capability of the mount by allowing a helix to be processed and operated at higher temperatures than helices supported in glass barrels.

In the past it has been found exceedingly difiicult to manufacture long, slender ceramic helix barrels of the proper dimensions to take advantage of the physical prop erties of ceramic, notably its strength at high temperatures. If the usual means are used, e.g., extrusion or broaching, distinct manufacturing problems arise.

In the former case, the ceramic must be extruded in its green or soft state. When the barrel is fired to effect fusion (hardening), non-uniform shrinkage occurs because of the inhomo-genous nature of the green ceramic. On the other hand, broaching operations necessitate the use of ceramic in the hardened state to effect a usable envelope. This ceramic is highly abrasive making this operation slow, costly and dependent upon operator skill.

A ceramic helix mount and envelope can be manufactured easily and at low cost, however, by the application of the principles of this invention using the steps of the method of assembly described above except for the elimination of the steps of inserting the helix support rods in the grooves of the ceramic tubular members. To fabricate this modification, two ceramic tubular members 152 and 154 shown in FIGURE 9 are first fabricated to pro vide internal radio frequency coupling to the helix and comprise an auxiliary metal outer conductor 159 and inner conductor 161 located coaxially with the assembly. In detail, metal tubes 159 are first machined to a precise inside and outside diameter, after which radial openings 171 are drilled in their outer surfaces and in the ceramic tubes. Cylindrical ceramic spacers 160 supporting within their respective tube bores 162 and inner conductor 161 having radial extending folded portions 174 are then located in bores 175 of the metal tubes 159 such that folded portion 174 projects through openings 171 of tube 159.

These assemblies in turn are slipped into the bores of ceramic tubes 152 and 154 and electrical connection between the inner conductor and the helix made. Similar steps as described in the previous embodiment are then followed except that, as previously noted, the step of inserting the rods in the grooves of ceramic tube pairs is eliminated.

Conclusions After the above fabrication steps of both embodiments are completed, a previously assembled gun structure comprising a cathode and one or more accelerating anodes is secured to the helix mount by inserting a tubular extension of the anode between the three rods 18, 19 and 20 or the walls of ceramic members 150, 151, 152, 153, 154, and 155 and then fastening, as by welding, the flat surface of the anode to end wall 108. Next, the gun housing 147 is joined to wall 108 by heliarc welding. Finally, rearward end wall 109 having openings through which insulated pins project making electrical connection to the gun elements, is joined circumferentially with gun housing 147.

It should be understood that this invention in its broadest aspects is not limited to the specific examples herein illustrated in that the following claims are intended to include all changes and modifications Within the true spirit and scope of the invention.

I claim:

1. In a traveling wave tube having a helix with a straight axis, a vacuum-tight envelope coextensive with said helix comprising a plurality of cylindrical members having axes parallel to said helix axis, said members being arranged in circumferential series coaxially of said helix with each of said members in tangential engagament with adjacent members, and means for vacuum sealing adjacent members together to form a vacuum-tight enclosure around said helix.

2. The structure according to claim 1 in which said members are rigid and said helix tangentially engages and is supported by said members.

3. The structure according to claim 1 with three cylindrical rods disposed between said helix and said members.

4. The structure according to claim 1 in which said members are tubular.

5. The structure according to claim 5 in which certain of said members comprise the outer conductors of coaxial transmission lines and have inner conductors, respectively, therewithin.

6. An envelope and mount comprising a helix having a straight axis, a plurality of first support members having a circular cross section disposed in tangential contact about and coextensive with said helix, a plurality of second support members coextensive with said helix having a circular cross section disposed in tangential engagament with at least one of first members, said first members having tangential forces applied coextensively with said helix on opposite sides of a plane passing through the axis of said helix and points of contact of said first members and said helix, and means for securing said first and second members in said engagement.

7. Structure according to claim 6 wherein first support members are three ceramic rods circumferentially spaced about said helix, and wherein said second support members are six tubular members.

8. Structure according to claim 6 wherein said second support members are six tubular members, a cuspal space being defined at the junction between adjacent members,

said first support member comprising three ceramic rods disposed in three circumferentially separated cuspal spaces, respectively, each of said rods being in tangential line contact with said helix and with two adjacent members.

9. In a traveling wave tube having a helix with a straight axis, an envelope coextensive with said helix comprising a plurality of cylindrical member-s having axes parallel to said helix axis, said members being arrang d in circumferential series coaxially of said helix with each of said members in tangential engagement with adjacent members and with the junction of adjacent members being cusp-shaped, three cylindrical rods disposed between said helix and said members, each of said rods being supported within one of said cusp-shaped junctions in circumferentially spaced relation with the other two rods, and means for sealing adjacent members together to form a vacuum-tight enclosure around said helix.

References Cited by the Examiner UNITED STATES PATENTS 2,650,350 8/53 Heath 333-31 2,730,649 1/56 Dewey 315--3..5 2,758,241 8/56 Robinson 3153.5 2,812,499 11/57 Robertson 315--3.5 X 2,845,690 8/58 Harrison 2925.14 2,869,217 1/59 Saunders 2925.14 3,020,498 2/62 Ash et a1. 333--31 X 3,062,983 11/62 Van Hoven 3153.6

ROBERT SEGAL, Primary Examiner.

RALPH G. NILSON, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,211,945 October 12, 1965 William H, Thon It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5, lines 17 to 19, the equation should appear as shown below instead of as in the patent:

column 7, line 33, for "spaced" read spacer column 11, line 52, for the claim reference numeral "5" read 4 Signed and sealed this 5th day of July 1966,

(SEAL) Attest:

EDWARD J. BRENNER ERNEST W, SWIDER Attesting Officer Commissioner of Patents 

1. IN A TRAVELING WAVE TUBE HAVING A HELIX WITH A STRAIGHT AXIS, A VACUUM-TIGHT ENVELOPE COEXTENSIVE WITH SAID HELIX COMPRISING A PLURALITY OF CYLINDRICAL MEMBERS HAVING AXES PARALLEL TO SAID HELIX AXIS, SAID MEMBERS BEING ARRANGED IN CIRCUMFERENTIAL SERIES COAXIALLY OF SAID HELIX WITH EACH OF SAID MEMBERS IN TANGENTIAL ENGAGMENT WITH ADJACENT MEMBERS, AND MEANS FOR VACUUM SEALING ADJACENT MEMBERS TOGETHER TO FORM A VACUUM-TIGHT ENCLOSURE AROUND SAID HELIX. 